An Update on Tiger Swallowtails in Ontario

AN UPDATE ON TIGER SWALLOWTAILS IN ONTARIO

Tiger swallowtail is a collective term used to describe several North American butterfly species featuring a yellow ground colour with distinctive black stripes. Two tiger swallowtail species occur in Ontario. The Canadian Tiger Swallowtail, Papilio canadensis (Rothschild & Jordan 1906), is found in northern and eastern Ontario while the Eastern Tiger Swallowtail, Papilio glaucus (Linnaeus 1758), is found in southern Ontario. These two species are very similar in outward appearance and historically, P. canadensis was thought to be a subspecies of P. glaucus. Prior to the utilization of molecular and genetic tools in taxonomy, species were defined primarily by observables such as wing pattern, flight times, host plant usage, etc. It is not difficult to see how these two very similar tiger swallowtail species could be classified as one. With advancements in technology and a better understanding of their life histories, it eventually became apparent that although these taxa look similar, they demonstrate consistent and significant differences, prompting the elevation of P. canadensis to full species status in 1991 (3).

For example, although both P. canadensis and P. glaucus can use black cherry (Prunus serotina Ehrhart 1784) and white ash (Fraxinus americana Linnaeus 1753) as larval host plants, P. canadensis strongly prefers quaking aspen (Populus tremuloides Michaux 1803) throughout its range while P. glaucus larvae essentially cannot survive on this host. Conversely, P. glaucus strongly prefers tulip tree (Liriodendron tulipifera Linnaeus 1753) and hoptree (Ptelea trifoliata Linnaeus 1753) as hosts, but essentially, these cannot be used by P. canadensis. When presented with 3-way choice tests in the laboratory using tulip tree, black cherry and quaking aspen, P. glaucus shows a strong preference for ovipositing on tulip tree and P. canadensis strongly

favours ovipositing on quaking aspen (9). Subtle wing pattern and shape differences are also seen between these two species which are discussed in more detail below.

In terms of overwintering, P. canadensis exhibits obligate diapause, meaning that there is one generation per year and larvae will enter diapause regardless of environmental conditions. However, P. glaucus exhibits facultative diapause, meaning that environmental conditions during the later stages of larval development (4th and 5th instars) dictate whether the resultant pupa will enter diapause (7). If late instar larvae of P. glaucus experience relatively long daylight length, the resultant pupae will develop directly into adult butterflies that emerge later that season. As daylight length decreases, a higher proportion of larvae will produce pupae which diapause and emerge as adults the following year. Somewhat surprisingly, temperature does not play a significant role in diapause determination (7).

With P. canadensis now recognized as a species in its own right, an interesting question arises: how do P. canadensis and P. glaucus interact where their ranges meet in southern Ontario and New England? For most biologically distinct species, no interbreeding occurs due to a variety of isolating mechanisms. However, these taxa are very closely related and hybridization can and does occur. This hybrid zone between P. canadensis and P. glaucus varies in width and shape depending on altitude, latitude and local microclimates, but has historically ranged from 41 to 44°N. In southern Ontario, this maps approximately to the Carolinian zone with the northern limit of P. glaucus occurring probably around the Greater Toronto Area. Over the last 3 decades, climate change and milder winter conditions have caused this hybridization band to move north by approximately 40 km, and simulation models show that a northward shift of 55-144 km can be expected per 1°C of warming (6).

Within this hybrid zone, there is a complex set of incompletely understood interactions including, but not limited to, climate, genotypic variation, mating preferences, host plant selection, and diapause characteristics. It is beyond the scope of this introductory article to detail our current understanding of these factors, but for the interested reader, a list of important references has been provided. For the typical amateur butterfly enthusiast, the most relevant and immediate consideration is how to differentiate P. glaucus, P. canadensis, and their hybrids in the field, based on factors such as location, flight times, wing pattern, and larval traits.

While subtle, there are a number of wing pattern differences between P. canadensis and P. glaucus. In Figure 1 (see back cover), the specimen on the left is a male P. canadensis, obtained from an egg found on quaking aspen in Montebello, QC on June 18 (and subsequently raised on quaking aspen). The specimen on the right is a male P. glaucus captured in Hamilton, ON on June 16. The middle specimen is a male hybrid raised from an egg found on black cherry on July 17 in Kingston, ON. Notice the larger size, more pronounced scalloping of the hindwing margins, and deeper yellow colour when P. glaucus is compared with P. canadensis.

Many of the diagnostic differences between these taxa are actually better seen on the underside. In Figure 2 (see back cover), the sizes of the specimens are not to scale in relation to each other. Proceeding from left to right, the first specimen is a female P. canadensis, captured just west of Edmonton, AB on June 15. The second specimen is a female P. canadensis from a 3rd instar larva found on chokecherry (Prunus virginiana Linnaeus 1753) on July 11 just north of

Kingston, ON. The third specimen is a hybrid female P. canadensis from an egg found on black cherry on July 17 in Kingston, ON. The last specimen is a female P. glaucus captured in Hamilton, ON on August 1.

Several features are noteworthy, as outlined by boxes 1-3. Box 1 shows a row of submarginal yellow spots on the front wing underside, which form a solid contiguous band in P. canadensis but are discrete lunules in P. glaucus. Box 2 shows the blue/black band on the hindwing underside, which is straight (or nearly so) in P. canadensis but scalloped in P. glaucus. Finally, box 3 shows the black band along the anal margin of the hindwing underside, which is much broader in P. canadensis than P. glaucus. Relative to the width of the entire cell containing the band, it is approximately 10-40% wide in P. glaucus and 55-90% wide in P. canadensis (12). Overall, the width of this black band is greater in females than males, but the relative difference between species persists (3). All these traits demonstrate variable intermediacy in hybrid specimens. Additionally, the interpretation of these findings should be tempered by the fact that spring brood P. glaucus specimens from the northern limit of its range can demonstrate P. canadensis-like traits (8), which is influenced by both genetic and environmental factors.

In the extreme southern tip of Ontario, one may rarely encounter a melanic form of P. glaucus which has a black ground colour with blue suffusion on the upper surface of the hindwings. This form is only seen in P. glaucus females, which are thought to mimic the distasteful Pipevine Swallowtail (Battus philenor Linnaeus 1771), another rare species occasionally seen in southern Ontario. Other than sporadic aberrations, melanic females are not known from P. canadensis. Intermediate individuals can be seen in the hybrid population (10).

Early instar larvae of these taxa are also different. In Figure 3 (see back cover), the larva on the left is P. canadensis from Edmonton, AB on quaking aspen. The larva on the right is P. glaucus from Hamilton, ON on tulip tree. The middle larva is a hybrid from Kingston, ON on chokecherry. All these larvae are in their 1st instar. Notice that the P. glaucus larva has a white saddle on its dorsal mid section while the P. canadensis larva has this saddle as well as anterior and posterior white bands. Hybrid larvae demonstrate reduced anterior and/or posterior white bands, such as the one depicted in the middle photograph.

The known ranges of Ontario tiger swallowtail species may also assist in their identification. South of a line from Hamilton to the southern edge of the Bruce Peninsula, pure P. canadensis is unlikely to be encountered, and any tiger swallowtail seen will be P. glaucus or P. glaucus with some genetic influx from P. canadensis. North of this line to the Greater Toronto Area, anything ranging from pure P. canadensis to pure P. glaucus may be seen, and it may sometimes simply be impossible to assign a definite identity to a specimen, especially if it is not captured and closely examined. Relying on location and flight times alone is not sufficient. North of the GTA to the Ottawa valley, it is unlikely to encounter pure P. glaucus. Based on historical climate data, north of this zone, the season is not long enough to support two generations of P. glaucus (11). If P. glaucus did exist in this region, any adults which emerge in May/June would give rise to larvae ready to pupate by mid summer. Without environmental cues to trigger diapause (they do not exhibit obligate diapause like P. canadensis), these pupae would develop directly into adult butterflies, leading to a second generation of P. glaucus larvae. Since the growing season is not long enough to allow this second generation to reach the pupal stage (or only rarely), there would

be intense selection pressure against P. glaucus. Thus, north of the GTA, we find P. canadensis or their hybrids.

Some sources state that north of the GTA region, a fresh tiger swallowtail seen in July or August can be considered P. glaucus, but this is not consistent with current research. These are late emerging hybrid tiger swallowtails and not a true second generation of P. glaucus. Consider fresh tiger swallowtails in July – there simply hasn’t been enough time for eggs laid in late May and June to have completed their life cycle yet to produce the July adults. In the literature, this has been termed a “false second generation” or “late flight” (LF) hybrid swarm (2, 11). The “early flight” (EF) tiger swallowtails emerging in May/June are P. canadensis with little to no genetic introgression from P. glaucus while the LF butterflies are hybrids. The latter also has only one generation per year, like P. canadensis, and produce offspring which overwinter and produce the LF adults of the subsequent year (Dr. Mark Scriber, pers. comm.). In Figure 2 (see back cover), the second specimen from the left is an EF P. canadensis with no definite P. glaucus traits. The third specimen from the left is a LF hybrid P. canadensis; notice the narrow black band along the anal margin of the hindwing, which is a P. glaucus trait.

Studies on the EF and LF adults have demonstrated differences in ovipositing preferences and ability to use different larval host plants (4). The LF hybrid adults favour laying eggs on tulip tree or hoptree when available, like P. glaucus. However, north of the GTA where these host plants are not often encountered (rarely planted as ornamentals), LF adults will typically lay eggs on white ash or black cherry. Hybrid larvae demonstrate survival on tulip tree, unlike pure P. canadensis, but this survival rate is lower than that of pure P. glaucus. Similarly, hybrid larvae also demonstrate survival on quaking aspen, unlike pure P. glaucus, but this survival rate is lower than that of pure P. canadensis (4). It is interesting to note that in the field, hybridization between P. glaucus and P. canadensis is not symmetric. Field experiments in Michigan and Florida have shown that both male P. glaucus and P. canadensis prefer to mate with female P. glaucus (1), although the reasons for this remain unclear. Yet, genetic analyses of these hybrids have found only P. canadensis-like mitochondrial DNA, suggesting that genetic introgression likely occurred via hybrid males mating with P. canadensis females (5). Future observations and further genetic/molecular studies will be needed to determine the exact nature of gene flow and degree of reproductive isolation between the LF hybrid population and EF P. canadensis in eastern Ontario.

Acknowledgements:

Dr. Mark Scriber, Professor Emeritus at Michigan State University contributed valuable personal commentary and copies of journal articles to this work. Thanks to Dr. Alan Macnaughton, Rick Cavasin and Dr. Laura Swystun for proofreading and feedback.

References:

1. Deering MD and Scriber MJ. Field bioassays show heterospecific mating preference asymmetry between hybridizing North American Papilio butterfly species (Lepidoptera: Papilionidae). Journal of Ethology 2002;20(1):25-33.

2. Hagen RH and Lederhouse RC. Polymodal emergence of the tiger swallowtail, Papilio glaucus (Lepidoptera: Papilionidae): source of a false second generation in central New York State. Ecological Entomology 1985;10(1):19-28. 3. Hagen RH, Lederhouse RC, Bossart JL, et al. Papilio canadensis and P. glaucus (Papilionidae) are distinct species. Journal of the Lepidopterists’ Society 1991;45(4):245-258. 4. Mercader RJ, Aardema ML, Scriber JM. Hybridization leads to host-use divergence in a polyphagous butterfly sibling species pair. Oecologia 2009;158(40):651-662. 5. Ording GJ, Mercader RD, Aardema ML, et al. Allochronic isolation and incipient hybrid speciation in tiger swallowtail butterflies. Oecologia 2010;162(2);523-531. 6. Ryan SF, Deines JM, Scriber JM, et al. Climate-mediated hybrid zone movement revealed with genomics, museum collection and simulation modeling. Proceedings of the National Academy of Sciences 2018;115(10):E2284-2291. 7. Ryan SF, Valella P, Thivierge G, et al. The role of latitudinal, genetic and temperature variation in the induction of diapause of Papilio glaucus (Lepidoptera: Papilionidae). Insect Science 2017;25(2):1-9. 8. Scriber, MJ. Interaction of introgression from Papilio glaucus canadensis and diapause in producing “spring form” eastern tiger swallowtail butterflies, P. glaucus (Lepidoptera: Papilionidae). The Great Lakes Entomologist 1990;23(3):127-138. 9. Scriber MJ. The inheritance of diagnostic larval traits for interspecific hybrids of Papilio canadensis and P. glaucus (Lepidoptera: Papilionidae). The Great Lakes Entomologist 1998;31(2):113-123. 10. Scriber MJ. Aberrant color patterns in the Papilio and an update on the semi-melanic “fletcheri” variants, including females (Lepidoptera: Papilionidae). Journal of the Lepidopterists’ Society 2009;63(2):118-126. 11. Scriber MJ, Elliot B, Maher E, et al. Adaptations to “thermal time” constraints in Papilio: latitudinal and local size clines differ in response to regional climate change. Insects 2014;5(1):199-226. 12. Scriber MJ and Ording GJ. Ecological speciation without host plant specialization; possible origins of a recently described cryptic Papilio species. Entomologia Experimentalis et Applicata 2005;115(1):247-263.

An Update on Tiger Swallowtails in Ontario (see article pg. 25)

Figure 1. Dorsal views of P. canadensis (left), hybrid (middle), and P. glaucus (right).

Figure 2. From left to right, ventral views of P. canadensis, early flight P. canadensis from within the hybrid zone, late flight P. canadensis from within the hybrid zone, and P. glaucus.

Figure 3. First instar larva of P. canadensis (left), hybrid (middle), and P. glaucus (right).

160